1. Field of the Invention:
The present invention relates to a variable capacitor, and more particularly, it relates to a variable capacitor which changes effective areas of electrodes on the basis of rotation of a rotor.
2. Description of the Background Art:
FIGS. 14 and 15 show a conventional variable capacitor 1 which is of interest to the present invention. Japanese Patent Laying-Open No. 5-299294 (1993), for example, discloses a variable capacitor which is substantially similar to this variable capacitor 1. The variable capacitor 1 shown in FIGS. 14 and 15 is of a surface-mountable type.
The variable capacitor 1 comprises a stator 2, a rotor 3 and a cover 4. The stator 2 comprises a ceramic dielectric member 5. The rotor 3 is made of a metal such as brass, and the cover 4 is made of a metal such as stainless steel or a copper alloy.
The stator 2 is provided with a stator electrode 6 in the interior of the dielectric member 5. External electrodes 7 and 8 are formed on both end portions of the dielectric member 5 respectively, so that the external electrode 7 is electrically connected with the stator electrode 6.
The rotor 3, which is arranged on an upper surface of the aforementioned stator 2, is provided on its lower surface with a substantially semicircular rotor electrode 9 which is defined by a projecting step portion, as clearly shown in FIGS. 16 and 17. The rotor 3 is further provided on its lower surface with a protrusion 10 which is flush with the rotor electrode 9, so that rotor 3 is prevented from inclination caused by formation of the rotor electrode 9. A driver groove 11 is formed in an upper surface of the rotor 3, for receiving a tool such as a screw driver for rotating the rotor 3. As shown in FIG. 14, the driver groove 11 has a "+" shape, for example, and is provided with a bottom.
The cover 4 is so shaped as to rotatably hold the rotor 3. An adjusting hole 12 is formed in an upper wall of the cover 4, to expose the driver groove 11. This cover 4 is provided with a pair of engaging segments 13 and 14 downwardly extending from opposite positions. The cover 4 is further provided with a downwardly extending rotor terminal 15.
The aforementioned cover 4 is arranged to cover the rotor 3, after a spring washer 16 is arranged on the upper surface of the rotor 3. The spring washer 16, which is made of a metal, is in the form of an annulus ring. End portions of the respective engaging segments 13 and 14, which are provided on the cover 4 are so bent as to approach each other, whereby these engaging segments 13 and 14 engage with the lower surface of the stator 2. The rotor terminal 15 comes into contact with the external electrode 8.
In this variable capacitor 1, the stator electrode 6 is electrically connected with the external electrode 7, while the rotor 3 provided with the rotor electrode 9 is electrically connected with the cover 4 through the spring washer 16 and the rotor terminal 15 provided on the cover 4 is electrically connected with the external electrode 8. Thus, capacitance which is provided by the variable capacitor 1 is derived by the external electrode 7 and the external electrode 8 or the rotor terminal 15. In order to adjust this capacitance, a screwdriver is inserted in the driver groove 11 and rotated in this state, thereby rotating the rotor 3. Effective opposing areas of the stator electrode 6 and the rotor electrode 9 are varied by such rotation of the rotor 3, thereby changing the capacitance which is formed across the stator electrode 6 and the rotor electrode 9.
During the aforementioned adjustment of the capacitance, the screwdriver is pressed against the rotor 3, not to be separated from the driver groove 11. Tests show that this pressing force is generally 100 to 300 gf. Such pressing force acts to press the rotor 3 against the stator 2, and temporarily narrows an extremely small clearance which can be defined between the rotor 3 and the stator 2. When the screwdriver is separated from the driver groove 11 after the adjustment, on the other hand, the aforementioned pressing force is removed and the clearance between the rotor 3 and the stator 2 is widened as compared with that during the adjustment.
It is well known that the capacitance is increased when the distance between the stator electrode 6 and the rotor electrode 9 is reduced, and vice versa. The aforementioned change of the clearance between the stator 2 and the rotor 3 changes the distance between the stator electrode 6 and the rotor electrode 9, thereby changing the capacitance. Therefore, the capacitance which may be obtained at a desired value by inserting the screwdriver in the driver groove 11 and rotating the rotor 3 is disadvantageously displaced from the desired value when the screwdriver is separated from the driver groove 11 (setting drift). When a pressing force of 300 gf which is is removed, for example, the capacitance is reduced by about 2%. Thus, it is difficult to adjust the capacitance of the variable capacitor 1.
When the screwdriver is manually operated, further, it is difficult to maintain the force which is applied from the screwdriver to the rotor 3 at a constant level. Thus, the capacitance is unstable during rotation of the rotor 3.